Let's play a game, shall we? I'm going to show you a sign, and I want you to make your best guess at what happened based solely on just a fragment of the picture.
Ready? Ok, here's a crop of the picture
What do you think happened at this place?
That's right, there was a shooting! The magic sign apparently had run out of fairy dust and unicorn dander thus somehow failing to prevent a goblin from ignoring the sign, entering with a handgun, and shooting the clerk to death.
While empty on fairy dust, it probably still contained enough leprechaun spunk to work its magic on the law abiding, who would follow the sign and not bring in their weapons, thus ensuring that said goblin could rest assured he'd not be thwarted in his efforts.
Here in the Gunshine State, signs like these have no legal power. If I were to walk into a business with a sign like that and get printed, they cannot arrest me or charge me with any crime simply for having the firearm. If the manager asks me to leave, I must, but that's regardless of having a firearm or not.
However, I do not give my patronage to businesses that consider me on par with criminals. I already have refused to do business with American Signature Furniture because they consider potential clients as potential murderers. Rarely will I break my rule of respecting businesses' property rights (and I admit I have). You don't want me as a client, I generally can find the same services or products elsewhere.
Besides, you never know if the management has remembered to fill up their no gun sign with Hopenchange.